New Zealand's longest serving prisoner - and one of the first to be jailed indefinitely - has been denied parole again.

And it is likely he will never be released due to deteriorating mental health and behaviour.

Alfred Thomas Vincent was one of the first people to be sentenced to preventive detention in 1968 after he was convicted of indecently assaulting five boys aged 12-14.

The child sex offender has spent more than 50 years in prison.

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READ MORE: Who has spent the most time behind bars?
Our longest-serving prisoner Alfred Thomas Vincent declined parole again

The only time Vincent has spent outside the razor wire was during day passes and weekend leaves in the early 80s - which were revoked when he was caught talking to young boys.

The Christchurch man is suffering from health issues to the extent there was no possibility for the Parole Board to "engage with him directly".

He was denied release from prison earlier this year and appeared before the Parole Board again on August 19.

Parole Board chairman Sir Ron Young said there had been a "possibility" Vincent could be released to a specific facility - even though he was displaying "inappropriate sexualised conduct" in jail.

But that was not an option now as his health had declined further.

"He is deteriorating both physically and mentally," Sir Ron said in a parole decision released to the Herald today.

"He continues poor conduct, invading personal space, some sexualised conduct, all of which may simply be disinhibited conduct arising from [his health condition].

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"It was noted that he might be able to be released to nursing home care but there were significant dangers in his sexualised conduct."

Sir Ron said a report provided to the board suggested that if Vincent were to be released to a "suitable psychogeriatric care facility" the staff may need to be trained to manage his sexualised behaviour.

"And the facility will need to be able to guarantee appropriate levels of observation and understanding of and response to Mr Vincent's tendency to seek out sexual contact with others," the report stated.

Sir Ron said there was no such care facility available in New Zealand.

"In the circumstances therefore, given Mr Vincent's continued conduct, we are satisfied he remains an undue risk," he said.

"We have made significant efforts since November 2018 to try and identify appropriate care facilities without success.

"None can now be identified.

"We think the appropriate course now is a rather longer period.

"We will see Mr Vincent again in just under two years' time ... If further enquiries as to appropriate care facilities prove fruitful then Mr Vincent can always come back before the Board for a further reconsideration of parole."